High-frequency filters for separating radiofrequency and multiplexers such as duplexers using the high-frequency filters are used for wireless terminals such as mobile phones and smartphones. For such filters and multiplexers, used are Surface Acoustic Wave (SAW) filters and Bulk Acoustic Wave (BAW) filters. Ladder-type filters in which one-port resonators are connected in a ladder form have been known as such filters. In the ladder-type filter, series resonators are connected in series and parallel resonators are connected in parallel between an input terminal and an output terminal.
To improve the attenuation characteristic in the blocking band of the ladder-type filter, it has been known to make the input terminal, the output terminal, and the impedance of the series resonators and the parallel resonators have a predetermined relation as disclosed in, for example, Japanese Patent Application Publication No. 2004-158970 (hereinafter, referred to as Patent Document 1). In Patent Document 1, the term “characteristic impedance” is used, but is not the original meaning of the word. To make input/output impedance high, it has been known to make the impedance of the series resonators and the parallel resonators high as disclosed in, for example, Japanese Patent Application Publication No. 2001-24471.
In the ladder-type filter, it is desired to improve the power durability and/or the linearity without deteriorating the insertion loss.